Risk Reduction

Abstract

The existing high alluvial risk Basin Lluta “Chile,” due to the presence of large mountains subject to inclement weather, coupled with the growing urban pressure to use sectors downstream increasingly exposed to this type of event, justify studies of the behavior of debris flows and the ability to mitigate its effects by the construction of protection and control. For this purpose, made the hidrogeomorfológica map for this basin. The surface of this basin is 3447 km2 and is in a desert area. It is characterized by low rainfall and therefore all its soils are devoid of vegetation except for the low part of the valley where the land use is agricultural. The natural regime has permanent basin runoff to the sea all year round with an average flow of 0.45–1.44 m3seg. The impact has increased the flow, but homogeneity is not presented with the full extent of the basin, showing notable spatial differences in the level of danger she originates. Different from what was described flood of 2001, which was reflected in the lower basin Lluta where road infrastructure was badly damaged by the effects of scouring and landslides, resulting in this reconstruction of road and railway bridges, implying a strong investment in works of river bank protection upstream of the bridges.

1. Introduction

The high alluvial risk existing in the Lluta basin due to the presence of large mountains subject to inclement weather, added to the growing urban pressure to use sectors of the lower basin that are increasingly exposed to this type of event, justify studies on the behavior of debris flows, and the possibility of mitigating their effects through the construction of protection and control works. For this purpose, the hydrogeomorphological chart for this basin has been prepared, being a management support tool, the main objective of this work.

The hydrographic basin of the river Lluta and its valley of the same name is located in the XV Region of Chile, comprised between parallels 18° and 18°30’ South Latitude and meridians 70°20 and 69°22’ West Longitude, in the Provinces of Arica and Parinacota, whose main river flows into the Pacific Ocean in the coastal sector called Chacalluta Valley.

This basin, in a natural regime, presents permanent surface runoff to the sea throughout the year with an average flow of 0.45m³sec. at 1.44m³sec., the Lluta river has carved out a quite narrow and deep valley, which is limited by quite steep and high slopes. The high alluvial risk existing in this basin due to the presence of large mountains subject to inclement weather, added to the growing urban pressure to use sectors of the lower basin, justify studies on the behavior of debris flows, and the possibility of mitigating their effects through the construction of protection and control works. The impact of the increase in flow does not occur homogeneously throughout the basin, showing notorious spatial differences in terms of the level of danger that it causes. Totally different from what was described as the 2001 flood, which manifested itself in the lower Lluta basin, where the road infrastructure suffered great damage due to undermining and landslides, resulting in the reconstruction of road and railway bridges, implying a strong investment in fluvial protection works on the banks upstream of the bridges, that is, fluvial defense works were carried out in specific areas.

2. Application

There are numerous works that show the utility or value of the hydrogeomorphological chart in the treatment or study of water basins and the planning of defenses against unexpected floods. In Ref. [1, 2], in their document, “An index for the hydrogeomorphological assessment of fluvial systems” points out that fluvial dynamics is the key not only to the functioning but also to the ecological, landscape, and environmental value of fluvial systems. If you want to conserve a river as an ecosystem and as an environmental corridor in the territory, first of all, its hydrogeomorphological dynamics must be protected, because this is what will guarantee the protection of each and every one of the elements of the system and its relationships. For this reason, the evaluation of the hydrogeomorphological functioning of river systems is essential to determine their ecological status, as well as their foreseeable trends, for which three evaluation parameters are considered:

• functional quality of the river system, including

  1. naturalness of the flow regime,

  2. availability and mobility of sediments, and

  3. functionality of the floodplain;

• channel quality, including

  1. naturalness of the layout and morphology in the plan,

  2. continuity and naturalness of the bed and longitudinal and vertical processes, and

  3. naturalness of the margins and lateral mobility;

• riverbank quality, including

  1. longitudinal continuity,

  2. width, structure, naturalness, and

  3. transversal interconnectivity.

Hydrogeomorphological cartography, as shown in Figure 1, is a document with particular characteristics aimed at being a tool that serves to mitigate water risks with the purpose of supporting the results of the application of a set of measures aimed at reducing risk and eliminating vulnerability, that is, physical, social, and economic. Thus, mitigation also constitutes one of the most important activities, since it allows actions to be carried out in advance, with the purpose of significantly reducing the expected consequences of an event. This stage is the most efficient and economical in terms of investment of resources and social cost, and of infrastructure and the environment.

Figure 1.

Hydrogeomorphological chart of the Lluta river, Arica and Parinacota region, Lluta river basin slope map.

3. Description of the upper section of the hydro geomorphological chart of the Lluta River

Each number of this section described in Figure 2 corresponds to the following:

1. Linear erosion

2. Creep

3. Medium infiltration capacity (fractured volcanic rock)

4. Permanent watercourse

5. Vegetables and bofedales

6. High infiltration ability (unconsolidated deposits-fill)

7. Order of strahler

8. Undercutting

9. Gullies

10. Very low infiltration capacity (sedimentary and mixed rocks)

11. Landslide niches

12. Slop

In the upper section, there are vegetation and bofedales, as shown inFigure 3, which in the same way are not enough so that the erosion processes are not activated. As shown in Figures 2 and 4, there is a permanent water course, which erodes material in this section, which is partly a consequence of its slope and there are arms around the permanent watercourse that reach the fourth order of Strahler in Ref. [3].

Figure 2.

Representation of the hydrogeomorphological chart of the Lluta River (high section).

Figure 3.

Boreal. Vegetal behavior near the Lluta river, which is partly the one found mainly in the upper reaches of the Lluta basin, as shown by the red circle.

Figure 4.

Representación de arrastre de rocas sedimentarias y mixtas en el flanco SE respecto al cauce principal del Río Lluta.

The river has a high infiltration ability (unconsolidated deposits-fill) far from the river’s flood bed. On the right flank of the Lluta river in this section, there is a part of land with a very low infiltration capacity (sedimentary and mixed rocks), since it corresponds to the deposit of material from the arms that reach up to the fourth order of Strahler. The river in its highest section presents in its flood bed a medium infiltration ability (fractured volcanic rock).

The upper basin is a complex climatic zone, characterized by intense cold and strong winds; which is added to the considerable height of the location and the great geographical distances that separate it from the main urban centers; Likewise, the geology of this study area has been determined in recent times, mainly by the predominant presence of units associated with the Tarapacá Volcanic Complex, whose products have their genesis in the volcanic events that occurred from the Pliocene to the Holocene, presented as stages evolutionary of such a complex. The eruptive events included processes of successive collapse of the volcanic building, pyroclastic flows, and lahars, among others in Ref. [4].

The type of permeability is primary in porous formation, and there are unconsolidated deposits, including fluvial, glacial, alluvial, lacustrine, alluvial, eolian sediments, aquifers of variable extension, generally stratified, free or semi-confined aquifers, variable permeability, and variable chemical quality…. Likewise, combined deposits or sedimentary rocks, such as sandstones, lutites, siltstones, and claystones from lacustrine or marine deposits, aquifers of variable extension.

Secondary permeability is also present in fractured volcanic rocks, consisting of lava flows, tuffs, and andesite breccias with intercalations of continental clastic sediment, rhyolitic and dacitic ignimbrite, poorly explored aquifers, of little-known extent and importance, attributable to aquifer characteristics of these formations, volcanoes of the Altiplano.

In the same way, permeability can be seen in an extremely low to absent condition in sedimentary and mixed rocks, sedimentary-volcanic rocks, lava flows, tuffs, gaps, ignimbrite with intercalations, shale, limestone, sandstone, and conglomerates; in general impermeable, they are considered basement of the aquifer fillings. Also, in plutonic rocks, such as granitic intrusive, impermeable basements, likewise, in volcanic rocks, lava flows and rhyolitic, dacitic andesitic, and basaltic pyroclastic deposits, associated with well-preserved or active ancient volcanoes, in general, they do not present aquifers characteristics.

The Tacora volcano of 6000 m above the nm., it is cataloged as an active volcano of the fumarolic type, with a stratovolcano structure, located 17°43’S; 69°46’W. As shown in Figure 5, an eruption of this volcano could generate lahars.

Figure 5.

Tacora volcano (source, photograph by Felipe Aguilera, University of Atacama, Chile). Main source of medium infiltration ability (fractured volcanic rock), as shown in red circle on the hydrogeomorphological chart.

Lahar phenomena stand for great danger mainly because they are natural phenomena of high energy, considerable destructive power, and are difficult to control. However, they are relatively easy phenomena to predict due to their movement in the direction that the valleys follow. The prediction of the time of occurrence of this type of phenomenon is not trivial, but normally the development of an eruptive event can be the antecedent to the development of this type of flow. The material could go to two main existing ones that are the Caracarani river to the east and the Azufre “Sulfer” river to the south of the slopes of the volcano.

Lahars are flows of volcanic materials generated when meteoric water, surface runoff, and partial snowmelt mixes with these materials and moves, transporting them en masse through the ravines and designated river channels that have their headwaters in the slopes of the volcano.

The Azufre “Sulfur” river found in the highlands, east, and south sector of the Tacora volcano, is close to the Peruvian border and its name is due to the high content of sulfur. It is a tributary of the Lluta river, as shown in Figure 6. In the upper areas of this river, there are volcanic rocks from the Miocene period.

Figure 6.

Photo of the slope of the Tacora volcano showing a fumarole. (Source, photo by Felipe Aguilera, University of Atacama, Chile). Likewise, the cause of unconsolidated deposits infill, which is dragged by the sulfur river, as reflected in the hydrogeomorphological chart.

The Azufre “Sulfur” river is formed from the meeting of several thermal springs, of which the Tacora stream is the main one, at the southwestern foot of the Tacora volcano. Its natural channel runs approximately 20 km in a SE direction.

The Asofre “Sulfur” River is currently diverted through an artificial canal to some evaporation ponds built on the Titire pampa, to avoid contamination of the Lluta. In the natural regime, the gauging practiced yield values of 30 l/s to 50 l/s as representative of the flow of the river as shown in Figures 6 and 7.

Figure 7.

Inlet in the Azufre river, to divert the polluted waters toward the “parapets” Humapalca locality, commune of General Lagos, Parinacota Province, Chile. As the hydrogeomorphological chart shows, with the representation of a red arrow, the Asufre river was one of the main tributaries of the Lluta, and regardless of its diversion, the old materials deposited form part of the hydrogeomorphological chart of the Lluta.

For this area, it is proposed to install buried tubes in Ref. [5], (Japanese defense type slit dam, and others) in the area of greatest probability of runoff into these channels, thus the lahar loses speed, dissipates energy, and delays its arrival at the channel, although only due to a sudden change in slope when reaching the base level of the flat areas on which the volcanic buildings rise, it stops, as long as the slope is gentle.

Continuing with the work of the hydrogeomorphological chart, the Caracarani river is analyzed, it originates at the foot of the Laguna Blanca pass and receives contributions almost from the borderline with Peru, from the eastern slope of the Tacora volcano 5966 m, from the new snow-capped from Chupiquiña 5787 m, and from the western slope of Cerro Caracarani 5190 m.

Due to the characteristics of the basin, the Caracarani River is considered the main tributary, from its source to Humapalca, as shown In Figure 8. It has a category 3 with low slope and on the sides to the course scour and creep processes can be seen, bearing in mind that pluviometric events, such as the one that occurred in the year 2001 (which have the probability of being generated possibly every 30 years), where the water fell during a period of 24 hours reached 20 mm. It may be of similar characteristics where the water fell on the entire surface of the basin in unison. However, for the conditions described and analyzed in the development of these points, it is pointed out that alluvial events can be generated by surface runoff. Likewise, it is necessary to increase the flow of the channels. For this, it is proposed to apply dams of gabion structures in this segment of the channel, whose purpose is to cushion the impact of the water flow in changes of slope and on the banks of watercourses. Thus, reducing its speed and filtering the sediments, that is, it allows regulating medium and large water flows, due to its great resistance in periods of flooding.

Figure 8.

View of the “parapets” of the Azufre River, from the channel that diverts the waters of the river to these ponds or pools. Town of Humapalca, commune of General Lagos. (source, Elias Munoz) As the hydrogeomorphological chart shows, with the representation of a red arrow, the Sulfur River was one of the main tributaries of the Lluta, and regardless of its diversion, the old materials deposited form part of the hydrogeomorphological chart of the Lluta.

The consumption of the Caracarani is very variable and follows the changes in ambient temperature, with an average of 400 liters/sec. The variations in the capacities show flows from 255 to 640 liter/sec.

The proposed application for this section is a dike or a sink, for its application, it must be excavated until an impermeable layer is found and if this is not found, the soil must be leveled and firmly compacted. The first row of gabions must be buried in the bed, then a second row is arranged on top of the first, although only at the ends, which make up the spillway toward the center. This run should be placed 25 cm further upstream than the first to increase resistance to the impacts of the floodwater flow in Ref. [6]. With this, it is possible to regulate water flow due to its great resistance in periods of flooding.

Likewise, in this channel, a protection system must be provided in curved sections due to the fact that the undermining in curves causes greater depths and speeds in the inner part of the same and, as a consequence, that the fluvial courses move laterally. This form of undermining is of great importance due to the serious damage it causes. The most usual forms of direct protection of the outer edge of a curve are groynes and marginal revetments or protections. With both solutions, the high-speed current lines move away from the materials that form the shore and cannot be dragged.

Another option proposed for this stretch is to consider open dams (slit dams) for this river: permeable or semi-permeable dams are more recommended, since they do not hinder the ordinary dragging of the channel or the living organisms that travel along it long of him.

Advancing toward the south of the upper basin, applying the hydrogeomorphological chart, we find the Quebrada Allane: it is a sub-basin that drains an area of 219 km² and corresponds to the northern limit of the Pampa Cascachane, its bed is characterized by presenting a single channel with an average of slope of 5.8° and category 3, the bottom of the channel from its source to the first third downstream is flat, then it presents a V-shaped bottom, in most of its trajectory, it only presents a certain level of anastomosis in the last part of its course, just before delivering its waters to the Lluta river.

The Allane ravine falls into the Lluta from the east, its main tributary is the Colpitas river. The latter is born at the western foot of the pass of the seven turns, a site between the Luxone and Pacocagua hills. The Allane river contributes 50% to the Lluta in the year, that is, 0.53 m³/sec., while the Lluta at that point for the same type of year, carries 0.86 m³/sec. The bed of the Quebrada Allane is characterized by presenting a single channel in most of its trajectory, it only presents a certain level of anastomosis in the last part of its course, just before delivering its waters to the Lluta river, in Ref. [4].

In the vicinity of the Quebrada Allane, terrace escarpments are located between the glacis and the beds of the Lluta river and the Quebrada Allane; deep terrace escarpments are developed that reach an average height of approximately 100 m in the Lluta. Slide scars can be seen on the upper edge of the escarpment. At the base of these forms, numerous gravity cones develop. All these forms testify to the erosive processes that the escarpment is currently undergoing. The origin of this form is due to the dissection that the Lluta has carried out as a pre-existing river in the landscape in the face of the continuous uplift that the area has experienced since the middle tertiary in Ref. [7], giving rise to the encasing of the water course, in Ref. [8].

Therefore, as in the segment of the Caracarani river, as shown in Figure 9, the use of dikes with gabion structures is proposed, as well as the application of breakwaters, which are structures supported or embedded in the shore that are within the current, which divert the current lines away from the shore. In addition, it favors the depositing of materials carried by the river between them. Depending on the case, they can be simple construction solutions and, therefore, economical. The cost of its maintenance decreases over time. Even with the tip of a jetty eroded, the rest of the structure continues to work, and the destruction of one of them does not seriously endanger the others. Notwithstanding the foregoing, periodic maintenance is required to ensure its permanence over time and enable simple repairs.

Figure 9.

Fluviometric station of the general Directorate of Waters of Chile, on the Caracarani river, near the town of Humapalca, commune of General Lagos, Parinacota Province. The Caracarani river is one of the tributaries of the Azufre river and, consequently, part of the deposits that it drag downstream are part of the results of the Lluta basin that are reflected in the representation of the hydrogeomorphological chart.

This course, which presents an area of gullies and slip niches, proposes the coatings or marginal protections that rest directly against the embankment of the shore and the bottom of the channel. These are parapets, and these are built with materials that cannot be carried away by the current. Between these materials and those of the edge, a filter is generally placed that prevents fine particles from escaping between the gaps in the marginal protection. Their main advantage is that they fix the shore definitively, without allowing any subsequent displacement following any curvature or configuration, although they require a more complicated and precise construction procedure than groynes; and, therefore, higher cost, in addition to requiring careful maintenance, since a failure, even of a small portion, endangers the entire protection structure.

For channeling work in the sectors with the highest speed (greater than 3 m/s) and in which stability and water tightness must be ensured, a reinforced parapet system is proposed based on a core of geotubes, in which the gravitational component is given by a geosynthetic tube filled with fine gravel or sand depending on availability.

From the data collected, it is pointed out that the river has very pronounced floods and that they develop rapidly, that is, the water level rises rapidly so that the runoff phenomenon is more assimilated to alluvial flow. This means that the work, in addition to resisting the erosive and dragging effect of the protection material, must consider the impact effect that a wave of these characteristics has on the work in Ref. [9].

Then, parallel to the Quebrada Allane, as shown in Figures 10 and 11, is the Quebrada de Colpitas. In this section, the slope processes are activated due to a steeper slope, these processes are creeping, undermining, gullies, slip niches, and linear erosion are located in the upper third of the development of the Lluta. The course of the Colpitas river, as shown in Figure 10, drains an area of 219 km² and has a 5° slope and is category 3. The bottom of this riverbed in its first third from its source downstream is flat and then its bottom is V-shaped. Like the Quebrada de Allane, the Quebrada de Colpitas, as shown in Figure 9, presents an area of gullies and landslide niches, for which the intervention in this course must be similar to the course of the Quebrada de Allane, as shown in Figure 12. Therefore, the application of defense is of the same characteristic as those indicated in the previous course, or else build alluvial control structures.

Figure 10.

View of the Quebrada de Allane in the background of the Taapaca volcano. The road passes over the right flank of the volcano in the image, the highest point is 4802 m. the union of the Quebrada de Allane and Colpitas can also be seen, where rock formations with different colors and shapes can be seen, with heights over 200 meters and vegetation conditioned to the highland desert climate. It is located at 3900 m.a.s.l. and distance of 3 km from the hamlet of Alcérreca. The Colpitas river is one of the tributaries of the Azufre river and, consequently, part of the deposits that it drags downstream are part of the results of the Lluta basin that are reflected in the representation of the hydrogeomorphological chart.

Figure 11.

The Allane ravine seen from the west. Contributions in material downstream, for the Lluta basin.

Figure 12.

Descent to the ford over the river Humapalca (Umapalca), on the road that joins the town of coronel Alcérreca (Alzérreca) and the town of Ancolacane. The Humapalca river as the cause of the current formation of the Lluta basin.

An important point to consider is through the general vision of the affected areas, where the spatial distribution of all the works is carried out, each one of them generates a protection zone on which its implementation will have a direct impact. Likewise, it is important to consider an interrelation of the works, which determines a correct functioning and fulfillment of the objectives. For example, in the control of gullies, the mere provision of dikes inside gullies without control of the runoff that causes this problem will be insufficient to stop its advance, so that at the head, at a distance greater than three times the depth of the gully in Ref. [10], a runoff control system must be available, by means of infiltration ditches or diversion channels among other possible alternatives to use.

It must be borne in mind that the gullies, sinkholes produced in rocks, and soils in sloping places due to the rainwater floods causing the so-called remounting erosion, contribute to the formation, normally, of ravines formed in the soft materials by the water of stream that, when there is a lack of sufficient plant cover, attacks the slopes digging long furrows with sharp edges.

The area’s climate is characterized as a humid high-altitude steppe, with rainfall and temperatures highly influenced by geographic height. In the study area, annual rainfall reaches an average of approximately 300 mm, and these are concentrated mainly in the summer months, between December and March.

The base runoff of the Taapaca creek is produced mainly by the melting of the Nevados de Putre. Therefore, its higher average daily flows must occur in the summer months, and the gauged ones obtained during winter times are lower than those of summer. The maximum flows in the ravine are produced occasionally by the floods produced by the summer rains of the Altiplano winter (Figure 13).

Figure 13.

Colpitas river during the highland winter rains of 2012. The Colpitas river is one of the tributaries of the Azufre river. So, part of the deposits that it carries downstream are part of the results of the Lluta basin that can be seen reflected in the representation of the hydrogeomorphological chart.

The ravine is characterized by a well-formed channel with some very confined areas that show the sporadic occurrence of important flows, as shown in Figure 14. Its bed is, in general, stony with sectors with a high content of sand.

Figure 14.

Taapaca ravine water scheme as part of the Lluta river hydrographic basin.

The flow measured at the confluence of its two headwater streams, in the Ruinas de Taapaca sector, averages 14 l/sec is increased to 30 l/sec. In the sector of the corrals downstream of Quebrada Milluni, due to the contributions coming from the affluent streams on the right side of the channel. The average flow measured is 30 l/sec. At the road crossing, it was estimated to be similar to that observed 1.3 km downstream.

Considering that the runoff of water in the Quebrada is produced by thaws, during the night there is no runoff, and it begins in the upper parts during the first hours of the day. In the corrals sector, a water front advance speed of 600 m/hr. has been measured, which decreases to 500 m/hr. downstream of this road, probably due to the decrease in the slope of the channel. Also bearing in mind that the Corrals sector is about 5–6 km from the confluence with the Allanes creek and, based on the data presented above, it is estimated that the waters would not be able to travel the distance to the Allanes creek before freezing or infiltrate, that is, the Taapaca creek in the area adjacent to the confluence with the Allanes creek.

Regarding eventual effects on the flora and vegetation of the sector, this is not sustained by the low flow that runs through the Taapaca creek, but, like all the vegetation of the Lauca national park, by the local rainfall regime.

Considering the characteristics of this ravine and its water contributions, background information obtained at the road crossing through the ravine approximately 1 km from its outlet to the Allane ravine, no water runs off and it is observed by dragging material and undermining, that the ravine has eventually runoff of high flows, which are attributed to floods resulting from the rains associated with the so-called Altiplanic winter.

The ravine is characterized by a well-formed channel with some very confined areas that show the sporadic occurrence of significant flows, much higher than what was observed. The characteristic of the flat channel bottom, its bed is, in general, stony with sectors with a high content of sand.

Therefore, only in the event of events, such as the one that occurred in 2001, this ravine would generate a contribution that would increase the bed of the Allane ravine. Therefore, to prevent an event of that magnitude or greater, it is proposed for this ravine to materialize the application of a control technique for floods, such as that affected by flood control works.

Regarding eventual effects on the flora and vegetation of the sector, this is not sustained by the low flow that runs through the Taapaca creek, but, like all the vegetation of the Lauca national park and surrounding area, by the local rainfall regime.

An important point of the analysis carried out in this area is to keep in mind the Tarapacá volcanic complex as defined by Clavero et al., in Ref. [11], this has historically been considered an inactive complex. However, the Spanish arrived in this part of the Andes, only about 450 years ago. In Ref. [8], the new data indicates that the Tarapacá volcanic complex is a dormant volcano, as shown the Figure 15, with strong potential for a future eruption. According to the spatial distribution of the domes and deposits in the Late Pleistocene-Holocene period, it is probable that, in the event of a renewal of activity, the products could be distributed on the S and SW flank of the complex. An injection of a new magmatic pulse could cause a deformation of the building, resulting in instability, and trigger a partial collapse of the upper flanks and domes, resulting in a debris avalanche toward the SSW. This sequence of events has been recurrent in stage IV (Late Pleistocene-Holocene, 0.45 Ma-Recent). In the case of an extrusion, explosion, and collapse of a dome, a pyroclastic flow can be caused, as occurred in the last stage (IV) of this volcanic complex.

Figure 15.

The Taapacá volcanic complex, stage II products (early late Pleistocene, 1.5–0.5 ma) and stage III (late Plesitocene, 0.5–0.47 ma) (source, manganese Los pumas environmental impact study, SGA 2011). Material from the Taapacá volcanic complex, is part of the Lluta basin deposit, as shown by the red circles.

In this regard, two possible scenarios could occur, the first of which could occur due to the development of an eruption between April and November, when there is a predominance of snow layers in the upper part of the complex as shown in Figure 15. Lahar flows may develop and probably be confined by the main valleys on the S and SW flanks of the complex, even affecting the international road that skirts Putre. Other flows could be caused in the direction of the western flank, affecting the main road that connects Putre with the towns that are further north in the Altiplano.

The second scenario, not associated with new volcanic activity, could develop during rainy seasons, from December to March. The rains induce the movement of small lahars each year, because of the remobilization of loose volcanoclastic material from the flanks of the complex. Although they rarely affect populated areas, they can often cut off roads. However, if an eruption were to occur, the availability of loose material could increase, and thus the volume of these flows could increase, favoring large discharges of material.

According to these scenarios shown through a statistical analysis extracted from the document in Ref. [8], on the Taapacá volcanic complex as shown in Figure 15, the volcanic eruptions in the last 30 ka, it is possible to say that the rate of eruptions is 1.6 per thousand years, with a recurrence of 450 years (95% confidence interval). The probability of occurrence of an eruption in the next 100 years can be estimated at 15%, and in the next 500 years, at about 56% in Ref. [8]. Considering the eruptive history in the late quaternary and its composition, another eruption can be expected in the next few years (decades).

Continuing with the description of Taapaca, the two main formations are the Taipicagua and Chilcacagua streams. Further down, the Asiruni and Ancachi streams are tributary. In the surroundings, there are nine main springs whose waters irrigate different sectors of the Andean oasis.

Extending the application of hydro geomorphological mapping, from the point where the Azufre river and the Caracarani river meet and take the name Lluta river, it is seen in this sector of the Lluta riverbed and surroundings of the town of Putre in the direction of the Tacora volcano, outcrop mainly from stratified rocks and semi-joined deposits. Unconsolidated sedimentary deposits occur in a very minor proportion. More than 50% of the rocks and semi-merged deposits correspond to deposits of pyroclastic flows, lavas, and ignimbrites tuffs, mainly caused by Plio-Pleistocene volcanic activity. Older volcanic rocks, from the Miocene, outcrop 20 km to the west and about 30 km to the east of the Putre locality area.

Also, complementing what was shown above, a bibliographic compilation of theoretical-technical and contingency background was used, related to the type of study and area studied, supporteds by a visual interpretation of the surface processes, based on satellite images Landsat, SPOT, ASTER, and obtained from the Google Earth program. Plus, and topographical and geological charts, the result of which was to complement the identification of dynamic geoforms or quaternary deposits (ephemeral channels, gravitational forms, current, recent alluvial cones and fans, types of perennial channels, slopes, mass slides, etc.)

Bearing in mind that the Lluta has a category 4 in the initial stretch in the first 10 km approximately and the bottom of the river is flat and then it changes to a V-shape. At an approximate distance of 18 km to the north of the town of Putre, there are mass removal deposits, and about 30 km north-northeast of this locality, there are fluvioglacial and moraine deposits. In addition, this is an area that has medium infiltration ability, also, adding to this is the precipitation rate for topographic elevation ranges, which is what it shows, which could be the water supply.

In the Lluta study segment in the upper basin, monitoring data from two fluviometric stations have been shown on the Colpitas river in Alcerreca and the Lluta river in Alcerreca. For each of these stations, graphs of the monthly averages and the annual averages have been prepared. There is not a very marked seasonality; however, there is a tendency to register higher flows in the period from July to September.

For the design of the works to be considered, a rainfall analysis is previously needed to estimate magnitudes and intensities of rainfall in short periods of time (less than 24 hours, generally less than or equal to one hour), which directly affect the size of the works to be built. With this information and with the description of the characteristics of the land, the runoff that each conservation work must control (critical runoff) is determined, in ref [6], thus designing a diversion channel or infiltration ditch capable of transferring or retaining, respectively, in a safe way said runoff to receiving.

Although we understand that these data are only a conceptualization of reality, they show us the importance that this factor exerts on erosion, in the same way, the relevance of being able to reduce any of the characteristics of the slope, which will allow the deposition of the particles. Of soil displaced by the action of water.

From Humapalca to Allane, the route is about 26 km to the SE, and along this route, the Lluta receives from the northeast the Chuquiananta and Guancarane ravines that are born at the foot of the snow-capped mountains of those names and that are of little significance.

About 10 km upstream from point Allane where the ravine of that name, the river that flows into the ravine of that name, converges, the Lluta river begins to deepen its course, which came higher up at the level of the plateaus, to form a great canyon carved in the rhyolitic tuffs and in other little cohesive sediments that follow it downwards. Thus, in front of the Coronel Alcérreca station, a height of no less than 300 m can be seen between the box of the plateau and the bottom of the river.

Continuing to the south, in the areas close to the Lluta riverbed on its sides, we find processes of slopes, such as landslide niches, an unstable zone, which slides with respect to a stable zone, through a surface or thin strip of land. Slippage occurs when the maximum tangential stress is reached at all points in the strip. For this type of slope, the blocks tend to roll and a vertical wall is required next to the trench so that the blocks do not try to get out.

The Lluta river continues in an increasingly narrow canyon heading north-south. In the next widening, called Jamiraya, the Putre ravine joins it on the left, which is born from the meeting of several watering holes originating in bofedales at the foot of the Nevados de Putre or Taapaca, as show the Figure 15. The gauging practiced in the Putre river in July 1968 showed a flow of 324 liters/sec for said cause in Ref. [12]. Shortly before the Putre junction, the Ancolacaya creek flows into the Lluta, parallel to it, with a very small contribution, estimated at less than 30 liters/sec.

The riverbed unfolds in a north-south direction, presenting an anastomosed pattern, with numerous diversions of channels between banks, mainly gravel together with accumulations of sand and gravel. Currently, the river has slightly affected its own bed in the sector, forming a small fluvial terrace, so the river box contains the bed itself, with a narrow terrace and a large number of gravity cones.

Analysis of the geological and geomorphological characteristics of the derived area in the analysis of average slopes, see slopes map, as shown in Figure 16, generating the identification of the potential activity of the geoforms, helping to determine the morphodynamic thresholds based on the slope ranges and their erosive potential. It can be considered that, according to the average slope values, all those slopes greater than 20° are recognized as potential generators of mass movements, and already over 30° of the average slope, as areas with high morphodynamics diversified in multiple processes, predominating the gravitational, in Ref. [8].

Figure 16.

Lluta river basin slopes map.

It is also due to considering what is related to the precipitations in the area that have an annual average value of 200 mm, while the evapotranspiration is 2000 mm. This makes the water balance in the soil very negative and infiltrations into the subsoil are minimal, limited only to precipitation events concentrated in very short periods of time.

4. Description of the middle section of the hydrogeomorphological chart of the Lluta river

The origins of the upper section of the Lluta river basin have been explained in detail, considering its foundations in terms of hydrographic and hydrological factors, which continue its journey downstream. Knowing the origins of its behavior and characteristics, the result of this is synthesized in the middle and low sections.

Each number in this section described in Figure 15, corresponds to the following:

1. Medium Infiltration Capacity (fractured volcanic rock)

2. Undercutting

3. Sliding Niches

4. High infiltration capacity (unconsolidated deposits-fill)

5. Linear Erosion

6. Very Low Infiltration Capacity (Sedimentary and Mixed Rocks)

7. Temporary watercourse

8. Vegetation less than 25%

In this section, it is joined, always on the left, by the Socoroma ravine, the last contribution of surface water that the Lluta receives. It is born in the snowless cordon that divides the Lluta basin from the Seco river basin to the south, a tributary of the San José river. In Coca, the last place of cultivation, the excess expense of spills and filters totals 25 liters/sec (Hydrographic Holes of Chile I Region of Tarapacá DGA).

Starting from Dos Hermanos, 77 km from the sea, the valley decreases its slope and widens. It begins in Chironte, 73 km from Chacalluta, the middle and lower course of the river with Agriculture Valley as shown in Figure 17.

Figure 17.

Representation of the hydro geomorphological chart of the Lluta river (middle section) (source, from the author).

Between Chironta and Boca Negra, several streams that are usually dry fall into the Lluta, but tend to grow with the summer highland rains. The main ones on the right are those of Chironte, in the sector of that name, and Palmani in the Vila Collo sector. Further down, on the south side, the Lluta receives the Chaquire ravines in Chaspisca and even further below Molinos, the Boca Negra ravine. Both are born in the Sierra de Huaylillas. From this point downwards, the Lluta increases with spring waters. From Boca Negra, the valley widens and the slope decreases to a value close to 2%.

In order to estimate the existing risk of flooding in the event of a major hydrometeorological event, the maximum flows have been obtained for return periods of 5, 10, and 20 years. For this, data from the fluviometric stations corresponding to Lluta in Alcérreca and Colpitas in Alcérreca have been used.

In the middle section of the Lluta river, as has already been mentioned, there is the consequence of the active processes in the upper section of the basin, large material that has been dragged, eroded, and undermined until reaching the middle section. It mainly constitutes an area for depositing sediments, which have been increasingly physically, chemically, and biologically weathered upon reaching the middle section, so the material arranged in this section is mainly in boulders.

This is reflected in the hydro geomorphological chart with an extremely low infiltration ability (sedimentary and mixed rocks) in the riverbed as such. After the bed riverbed, we find medium infiltration ability (fractured volcanic rock). And a high infiltration ability the further away from the bed riverbed.

Vegetation of less than 25% is found, which decreases with respect to the high section. As there is less vegetation in this area, it prevents the normal flow of surface runoff from being lost, which is why the normal processes of slopes are triggered, such as linear erosion, undermining, slip niches, producing dragging of material toward the lower section of the river and, due to the force of scour and drag with which the surface runoff comes from the upper section, temporary watercourses are generated.

Already within the analytical description, in the area of influence, it was determined that the western sector of the course of the Lluta near Putre is the one with the greatest slopes. In general, the horizontal, gentle, and moderate slopes are associated with the bed of the Lluta river, the colluvial glacis, and the volcanic terrace, see the Map of slopes. In these last two, the sediment movements are conditioned to the dynamics coming from precipitation as it drains on the surface as sporadic laminar flows.

The area of acute danger is identified as all those forms that show current activity: the active valley slopes and the excavation slope, whose slopes are complex (great gradient variability, i.e., between 50 and 100%) and that are potentially exposed to events of colluvial origin, such as rock falls, debris flows, translational and rotational landslides, and complex movements, with great granulometric variability, from fine to coarse.

The area of intermediate danger, are all those that can border with areas of acute danger, being forms of ancient or recent origin and that can present slopes of 5 to 20°. Its risk is medium due to the potential danger that its immediate contiguity offers to slopes with acute risk, which through mass movements, such as debris flows, rock falls, rotational or translational landslides, and complex movements, feed the slopes making the escarpment It is considered that these phenomena will be less recurrent than the areas of acute danger.

5. Description of the lower section of the hydrogeomorphological chart of the Lluta river

The lower section of the Lluta river basin is a reflection of the normal process of the dynamics of a fluvial course since it is here where the processes activated with force and by the effect of gravity settle in the upper section, continuing in the middle section, and culminating in the lower or lower section of the river.

Each number of this section described in Figure 18 corresponds to the following:

1. Linear Erosion

2. Sliding Niches

3. Temporary Watercourse

4. Creep

5. Permanent Watercourse

6. High Infiltration Capacity (Filled Unconsolidated Deposits)

Figure 18.

Representation of the hydrogeomorphological chart of the Lluta river (lower section). (Source, Author’s own).

In this section of the river, a high infiltration ability predominates (filling unconsolidated deposits), around which there is a permanent watercourse, as shown in Figure 2 to next to the riverbed. In the lower section of the river and in the bed itself, there is a medium infiltration capacity ability.

Slope processes, such as linear erosion, slip niches, and creep, are activated to a lesser extent in the upper and middle reaches of the river, since here the materials that were dragged in the processes activated upstream have been deposited.

A lithological control is predominant toward the right bank of the river, where at the same time there is a depositional margin, and where at the same time the creep slope process is activated as shown in Figure 19.

Figure 19.

Lithological control toward the left bank of the Lluta river, which shows that toward the right bank where a fluvial terrace has formed, there are creep processes. At the same time, on the right bank over said control, there are landslide niches.

The vegetation and bofedales in the lower section of the river are scarce, and the deposit of material dragged by gravity from the upper section predominates. In addition, facilities and equipment by human actions predominate in this area.

6. Conclusion

Considering the hydrographic basin of the Lluta river as a case study, it is naturally characterized by a marked imbalance of the water-soil-vegetation system, which, associated with human activities and extreme hydrometeorological phenomena, gives it a torrential hydrological behavior that causes significant drag. With devastating effects of solids with alluvial load, which affect extreme damage to the lower part of the Lluta valley and the mouth of the beach area of the city of Arica.

Reflecting on the results of the analysis obtained for the entire basin in general, it can be concluded that it has a character of high torrentiality and susceptibility to erosion due to the basic conditions of the relief and the drainage system. The behavior regarding about the hydrological response of the basin is fundamentally determined found by the morphometric attributes of the catchment basin, in the upper course of the Lluta river. Thus, the middle and lower basin are very strongly influenced by the hydrological behavior of the upper basin.

In relation to the part and process elements studied, the hydro geomorphological chart is presented as a very complete tool, although it has deficiencies. The great dynamics offered by hydrographic systems require this type of cartography, which must be subject to a periodic updating system, updating its database, to like the process proven with regular digital cartography. Likewise, this product must be developed for all the basins susceptible to a process like the one presented, even more, considering the variables of climate change.